As one of a back frame structure of a vehicle seat, there is a structure in which a gate-shaped back frame serving as a framework of a seat back and having a substantially inverted U shape in a front view is formed by connecting upper end sides of a pair of side frames extending in an up and down direction with an upper frame in a bridging manner. Such type of a back frame disclosed in JP-A-2014-156212 is used for a three-point seat belt built-in type vehicle seat. In the back frame structure disclosed in JP-A-2014-156212, an upper end portion of a high-strength side frame on one side to which the load from the seat belt is directly applied and an upper end portion of a normal-strength side frame on the other side are connected by the upper frame, and a belt guide is attached to the upper end portion of the high-strength side frame. The belt guide is a member that guides a webbing released from a webbing take-up device of a seat belt device disposed inside the seat back so as to extend from the back side to the front side of the back frame.
In the above structure, when it is attempted to integrate the belt guide with the upper frame in order to reduce the number of parts, it is necessary to make the upper frames of different structures in the left and right seats since the attachment position of the belt guide differs between the left and right seats. Here, even when the upper frame is made of a fiber-reinforced resin or a light alloy, and the belt guide part is formed differently on the left and right sides by using one molding die as an insert structure of a molding die, it is inevitable to make the upper frames of different structures on the left and right when the shape of the upper end portion of the high-strength side frame is different from the shape of the upper end portion of the normal-strength side frame. The reason is that the upper end portion of the high-strength side frame has to be attached to the side on which the belt guide is disposed, and thus, the portion to which the upper end portion of the high-strength side frame is attached and the portion to which the upper end portion of the normal-strength side frame is attached also have to be exchanged at the left and right.